176 FRANCIS VILLY. 



The peculiar development of this organ in the lowest forms with 

 such a structure raises a question : Are the hyomandibular cleft and 

 Eustachian tube homologous ? If I am correct in stating that there 

 is no connection between the two organs at any period in the frog's 

 history, there is obviously strong reason for doubting the generally 

 accepted doctrine of their homology. But it may be said that the 

 frog's development is modified in this particular point, and that as 

 the two first visceral arches have a peculiar history, in which they 

 probably undergo great larval modification, so they have modified 

 the structure placed between them, i.e., the hyomandibular cleft. 

 The mandibular arch in its early movement, and the hyoid arch by 

 its articulation with the mandibular arch, might cause the hyoman- 

 dibular cleft to develope in a peculiar way, so that it disappears at 

 one time and again reappears. This is not impossible, for in the 

 second stage described the Eustachian tube occupies a position 

 between the mandibular and hyoid bars at their point of articulation, 

 and thence it runs forwards. Such a course would be peculiar for 

 the hyomandibular cleft; but when the relations of the arches 

 concerned are considered, it will be seen that the early position of 

 the Eustachian tube is not irreconcilable with the view that it is 

 morphologically the hyomandibular cleft. The second point in the 

 development of the tube, that is, its breaking up into pieces during 

 the metamorphosis, may possibly be explained as being due to 

 mechanical causes, originated by the movement of the hyoid and 

 mandibular arches at the time. As far as the considerations 

 advanced go, it is justifiable to hold that the frog's development 

 does not absolulutely disprove that the Eustachian tube and hyo- 

 mandibular cleft are homologous, although they are probably not 

 connected in development. This view would be in accordance with 

 the doctrine generally held, although a not unimportant section of 

 investigators have considered that actual embryology does not bear 

 out the doctrine usually taught. At the same time it should be 

 remembered that the evidence offered by the frog — and such evidence 

 should have great weight — tends to show that the two organs have 

 no connection whatever with each other. 



The Skeleton of the Ear. — In this section is included the development 

 of the annular cartilage, stapes, and columella. Of these the columella 

 is the only one that is of great general interest, from its supposed 



